sluice, flood gate, canal lock
牐 centers on a water-control structure: a sluice, flood gate, or canal lock that manages water flow.
牐 combines 片 (a split piece of wood, often used in structures) with 臿, which likely contributes the sound and relates to inserting or channeling. Together they suggest a wooden gate or sluice for controlling water.
The left side 片 suggests a flat wooden board, and the right side 臿 looks like a gate mechanism with a plunger. Picture a wooden sluice gate being lowered into a canal to control water flow.
For ソウ, imagine the sound of water rushing through a sluice gate: a deep 'soh' sound as the gate opens. Soh -> ソウ.